The present invention is directed to electromechanical battery flywheel systems for energy storage and power delivery. More particularly, the present invention is directed toward flywheel energy storage systems having a variable speed, counter-rotating containment vessel for mechanical containment of high speed rotors and managing net momentum and net external gyroscopic forces.
Generally, battery flywheel systems comprise a single high-speed rotor which is mounted on a central shaft and which is supported by bearings attached to each end of the shaft. The flywheel and shaft are often enveloped within a heavy stationary containment vessel which is generally evacuated to minimize energy losses. In such arrangements, the shaft bearings are attached to the end plates of the flywheel containment vessel and the rotor is driven by one or more motor/generators mounted to the end plate. It has been proposed that flywheel energy storage systems be used in terrestrial (e.g. electrical) and extra-terrestrial (e.g. satellite) vehicles. However, management of momentum together with substantial gyroscopic forces associated with many prior flywheel storage system designs has impeded development in this area. In a flywheel system, gyroscopic forces arise from prescribed angular rotation of the flywheel about directions not coincident with the flywheel spin axis. Such forces result when the motion of the vehicle in which the flywheel is mounted are imposed on the flywheel rotor. Thus, when a vehicle having a flywheel situated therein undergoes a change in direction, gyroscopic forces may result which are orthogonal to the imposed movement. The gyroscopic forces grow larger with the size of the flywheel, with the rate of change and direction of the imposed motion, and with the speed at which the flywheel rotates about its spin axis.
Several methods have been suggested to compensate for the external gyroscopic forces associated with flywheels. A first method involves gimbal mounting the flywheel containment vessel so as to avoid or minimize the external gyroscopic moments that would result if the containment cylinder were rigidly attached to the vehicle. Rigid attachment of a single flywheel to the flywheel containment cylinder results in direct transfer of the external gyroscopic forces to the vehicle. Although the gimbal mount is effective in preventing transfer of vehicle motion to the flywheel, the gimbal mount provides a relatively weak mechanical connection between the flywheel containment cylinder and the vehicle. In the event of flywheel failure, large forces and moments may be applied to the gimbal which could well exceed the strength of the mechanical connection of the gimbal. Further, some gimbal designs compensate for limited degrees of motion. If the motion of the vehicle exceeds the limited degree of motion for which the gimbal is designed to compensate, the gyroscopic forces are transferred to the vehicle. Thus, the mechanical limitations of the gimbal may preclude it in some applications from being a satisfactory solution to the problem of gyroscopic forces.
Another method for preventing external gyroscopic forces from being exerted on a vehicle having a flywheel therein involves utilizing two coaxial, counter-rotating flywheels or rotors as disclosed in U.S. Pat. No. 5,124,605, entitled, xe2x80x9cA Flywheel-Based Energy Storage Methods and Apparatusxe2x80x9d rather than a single flywheel. The application of two rotors also provides momentum management which is particularly useful in satellite operation when charging or discharging a flywheel system. The object of the multiple flywheel design is to counter-rotate two flywheels so as to control momentum and produce a net zero external gyroscopic force. In most such embodiments, two identical flywheels are mounted onto a single or separate shaft with each flywheel being driven by (and driving) a separate motor. The success of this method of preventing gyroscopic forces and managing momentum depends upon synchronizing the operating frequencies of the counter-rotating flywheels. Such systems still require heavy stationary containment vessels in addition to the multiple rotors in order to insure safe operation. Indeed, a shortcoming of prior flywheel systems in general is the need for heavy stationary containment vessels which offer protection during flywheel failure. In the event of a sudden failure of a flywheel rotor, the large angular momentum of the high-speed flywheel rotor can be rapidly transferred to a containment cylinder. In conventional flywheel systems, the containment cylinder does not rotate and is rigidly attached to the vehicle. Strong and heavy attachments are required to prevent angular motion of the containment cylinder during (and immediately after) a flywheel burst. Some prior systems have a second stationary inner containment vessel which is free, although not driven, to rotate inside the outer containment cylinder. Such prior art systems operate by imparting some of the energy dispersed during a flywheel failure onto the inner containment cylinder which is free to rotate and dissipate energy. Under a burst rotor scenario the flywheel angular momentum is transferred first to the inner cylinder and then to the outer cylinder and ultimately to the vehicle. Thus despite the use of multiple containment cylinders, the prior art does not adequately isolate the vehicle from reaction forces resulting during flywheel failure. Further, the use of multiple large cylinders for containment generally produces an overweight and impractical design for mobile deployment.
A farther shortcoming of prior flywheel systems is the inability to simultaneously provide adequate torque and power. Typically, in prior systems, a motor/generator is directly coupled to the high-speed energy storage rotor and is the only source of torque and power. Because currently available high-speed motor/generators are limited in torque and power capacity, the flywheel battery is likewise limited.
In general, the maximum torque and horsepower which a motor/generator an produce depends primarily on the physical size of the motor. The high-speed motors which are typically used in flywheel systems are necessarily small in size because the rotating elements of the motor must withstand the high rotational stresses produced by the very high rotational frequency of the flywheel. In contrast, more powerful motors are large in size and operate at lower rotational frequencies than current state-of-the-art flywheel rotors. The maximum torque which a high-speed motor/generator is capable of producing is limited by the interacting magnetic fields located within the motor. In other words, the maximum torque for a given motor is determined by the number of magnetic poles located on the motor as well as the strength, volume and mean diameter of the permanent magnets. A motor/generator with a large diameter has sufficient room for a greater number of magnetic poles than a motor/generator with a small diameter; therefore a motor/generator with a large diameter can be designed with a higher maximum torque capacity. Thus, for configurations where the high speed rotor and motor are directly coupled, the diameter of the motor/generator limits the torque capacity of the system.
The torque limitation of the prior art is important to applications which require large power transfer during charge and discharge from a single energy storage unit. One such application is a hybrid electric vehicle where the primary function of the flywheel is to provide peak power to the vehicle drive train when vehicle power demand exceeds the horsepower capacity of the internal combustion engine. Another such application where the torque limitation is important, is satellite control systems which use motor reaction torques for altitude control. Thus, there is a need in the art for a flywheel battery system which can provide broader torque and power characteristics.
Still another shortcoming of prior art flywheel systems is the suspension/drive systems. Prior flywheel system suspension designs, whether single or multiple rotor configurations, require that the primary bearings, which are generally magnetic but also could be mechanical, and the secondary bearings, which are generally contact bearings used in the case of failure of the primary magnetic bearings, be connected directly to the high speed rotor. Application of such bearings to high speed applications results in eddy currents and frictional losses both of which increase with rotational frequency. There is a need in the art for a low loss bearing suspension system which offers full support during high speed flywheel rotor operation but which minimizes energy loss and undesirable heat build-up.
Accordingly, there remains a need for a flywheel energy storage system that minimizes net gyroscopic forces, manages momentum, minimizes bearings suspension losses, minimizes forces transferred to the vehicle during failure of the flywheel rotor, and provides adequate torque and power. It is also desired to provide such systems with higher energy densities simultaneous with higher power densities. The present invention is directed to these, as well as other, important ends.
A flywheel system in accordance with the present invention overcomes the shortcomings in the prior art by providing an inner rotor and an outer rotor coaxially mounted, wherein the outer rotor substantially cylindrically surrounds the inner rotor. Each of the inner and the outer rotor are rotatable about the axis. Unlike prior flywheel systems which used multiple rotors just for gyroscopic and momentum control, the outer rotor operates as a containment vessel for the inner rotor.
According to one aspect of the inventive flywheel system, the inner rotor and the outer rotor counter-rotate about the axis. A preselected net momentum with resulting gyroscopic force can be generated upon counter-rotation of the inner and outer rotors. In one embodiment, an essentially net zero external gyroscopic force results upon counter-rotation of the inner and outer rotors. Generally, the inner rotor has an inertia relatively less than the inertia of the outer rotor but rotates at higher speeds. The flywheel system may also be enclosed in a vacuum.
The inventive flywheel system comprises a means for coupling and suspending the inner rotor and the outer rotor so as to manage the relative rotational velocities of the inner rotor and the outer rotor and thereby generate a preselected net momentum.
In one embodiment of the invention, the flywheel system comprises a drive assembly for coupling the inner rotor and the outer rotor. The drive assembly may comprise a plurality of drive wheels positioned parallel with the axis of the inner rotor at a radial distance from the center of the inner rotor. The plurality of drive wheels are movably interconnected with the inner rotor and the outer rotor whereby movement of the inner rotor is transferred through the plurality of drive wheels to the outer rotor causing the outer containment rotor to counter-rotate relative to the inner rotor. Alternatively, the drive assembly comprises the following items: a drive shaft, the inner rotor being integrally connected to the drive shaft; a plurality of bearing posts positioned parallel to the drive shaft at a radial distance away from the center of the drive shaft; and a plurality of drive wheels, one of the plurality being mounted on each of the plurality of bearing posts. The drive wheels are movably interconnected with the inner rotor and the outer rotor whereby movement of the inner rotor is transferred through the plurality of drive wheels to the outer rotor causing the outer rotor to counter-rotate relative to the inner rotor. The drive assembly may comprise the following items: a drive shaft, the inner rotor being integrally connected to the drive shaft; a shaft drive wheel rotatably mounted around the perimeter of the drive shaft, the shaft drive wheel rotating with the drive shaft; a force transfer ring integrally coupled to the outer rotor for transferring forces to and from the outer rotor, the force transfer ring rotating with the outer rotor; a plurality of beaming posts positioned parallel to the drive shaft at a radial distance away from the center of the drive shaft; a plurality of bearings movably rotatably mounted on the plurality of the bearing posts; and a plurality of drive wheels, one of the plurality being mounted on each of the bearings. The drive wheels are movably interconnected with the force transfer ring and the shaft drive wheel whereby movement of the inner rotor is transferred through the shaft drive wheel to the drive wheels, and from the drive wheels to the force transfer ring, thereby causing the outer rotor to counter-rotate relative to the inner rotor. The drive assembly may further comprise the following items: radial magnetic bearings operably coupled around the central shaft for maintaining the radial position of the shaft; and axial magnetic bearings operably coupled for maintaining the axial position of the shaft. The drive assembly may still further comprise the following elements: rotating touchdown bearings operably coupled to the central shaft for limiting flywheel excursions during shock loading; and a motor/generator for energy and power delivery.
In one embodiment of the invention, the flywheel drive assembly may comprise a drive shaft to which the inner rotor is integrally connected, and a planetary drive base which is substantially coaxially mounted with the drive shaft and rotatable about the drive shaft. The flywheel drive assembly further comprises a plurality of drive wheels rotatably attached to the planetary drive base. Each of the plurality of drive wheels is rotatable about its own axis and simultaneously rotatable about the drive shaft upon rotation of the planetary drive base. The drive wheels are movably interconnected with the inner rotor and the outer rotor whereby movement of the inner rotor is transferred through the plurality of drive wheels to the outer rotor causing the outer rotor to counter-rotate relative to the inner rotor.
In another embodiment of the system, the flywheel drive assembly may comprise an inner rotor rotatable about a first axis and an outer rotor counter-rotatable about a second axis which intersects on at least one dimensional plane with the first axis. The outer rotor substantially surrounds the inner rotor. A first motor/generator operably coupled to the inner rotor causes the inner rotor to rotate about the first axis, A second motor/generator operably coupled to the second rotor causes the outer rotor to counter-rotate about the second axis relative to the first axis. The relative net momentum of the inner rotor and the outer rotor is controllable by the relative rotational velocities of the inner and outer rotor.
In one embodiment of the invention, the flywheel system comprises the following items: a first motor/generator capable of relative high rotational speeds operably connected to the first rotor; and a second motor/generator capable of lower rotational speeds relative to the first motor/generator but having a relatively greater torque capacity than the first motor/generator, operably connected to the second rotor.
Thus, there is disclosed in a flywheel system for storing energy comprising a first rotor and a second rotor counter-rotating relative to the first rotor, the combination wherein the first rotor has a relative low inertia and high rotational velocity as compared to the second rotor and is situated substantially internal to the second rotor which has a relatively large inertia and lower rotational velocity as compared to the first rotor and wherein the relative rotational velocities of the first rotor and the second rotor are maintained by a mechanical and magnetic drive assembly.
According to another aspect of the invention, there is disclosed a method of operating a flywheel system comprising first and second rotors, the second rotor being located substantially within the first rotor and having a relatively smaller mass than the first rotor. The method comprises the following steps; rotating the first rotor; and counter-rotating the second rotor relative to the first rotor at a relative lower rotational velocity so as to produce a managed momentum balance between the first rotor and the second rotor. According to another aspect of the invention, there is disclosed a method for releasable storing energy in mechanical form. The method comprises the following steps: transferring the energy into each of an inner and an outer coaxially mounted counter-rotatable rotor, the outer rotor being substantially cylindrically surrounding the inner rotor; and rotating one of the rotors, while counterrotating the other of the rotor. In one embodiment, the counterrotation gives rise to a preselected net momentum balance. The preselected net momentum may be zero.
According to another aspect of the invention, there is disclosed a vehicle having a flywheel energy storage system comprising the following items: a first rotor; a second rotor located substantially around and counter-rotating relative to the first rotor and having a relatively large mass and lower rotational velocity as compared to the first rotor; a drive assembly integrally coupled to the first rotor and the second rotor for maintaining constant relative rotational velocities between the first and the second rotor so as to produce a managed angular momentum balance between the first rotor and the second rotor. In one embodiment the vehicle is a terrestrial vehicle.
According to another aspect of the invention, there is disclosed a minimal weight, maximum energy and maximum specific energy rotor. The inventor rotor comprises the following items: an inner rim; an outer rim; a spacer ring which is interference fit to the outer rim; a tapered transition section extending between the inner rim and the outer rim, the transition section attached to the outer rim at the spacer ring; a stiffness over-wrap surrounding the inner rim for securing the transition section to the inner; and a growth ring to assist in compatibility of radial deformation between the transition section, the spacer ring, and the rim.
The inner rim, outer rim, and transition section are preferably manufactured from a composite material. The transition section may have a varying thickness. A minimal weight, maximum energy and maximum specific energy flywheel system having a rotor, wherein the rotor comprises the following items: an inner rim manufactured from composite material; an outer rim manufactured from composite material; a spacer ring, the spacer ring interference fit to the outer rim; a tapered transition section extending between the inner rim and the outer rim, the transition section attached to the outer rim at the spacer ring; a stiffness over-wrap surrounding the inner rim for securing the transition section to the inner; and a growth ring to assist in compatibility of radial deformation between the transition section, the spacer ring, and the outer rim.